JP2012-219745A discloses as a conventional internal combustion engine provided with a cylinder block able to move relative to a crankcase, one provided with two eccentric shafts (cam shafts) arranged at the two sides of the internal combustion engine and one drive shaft made to rotate by an actuator and making the eccentric shafts rotate in opposite directions to each other to make the cylinder block move relatively. In this conventional internal combustion engine, further, to keep the cylinder block from tilting in a direction different from the direction of relative movement, one side surface of the cylinder block is pushed by pushing members (biasing mechanism) while the other side surface of the cylinder block is supported by support members.